This vehicle-controlled rodent study (n=16) compared the antidepressant and anxiolytic effects of ketamine (5, 10, 15 or 30 mg/kg) to the antidepressant imipramine (10 mg/kg), using chronic exposure to mild stress as a depression model and assessing their cognitive capacity of novel object recognition and their natural aversion to open spaces. Results indicated a sustained antidepressant-like effect of ketamine at an optimum dose of 10 mg/kg, which reversed the anxiogenic and dyscognitive effects of chronic mild stress exposure much faster than the classical antidepressant imipramine.
Abstract
“Introduction: Ketamine is the prototype of a new generation of antidepressant drugs, which is reported in clinical studies to be effective in treatment-resistant patients, with an effect that appears within hours and lasts for a few days. Chronic mild stress (CMS) is a well-established and widely used animal model of depression, in which anhedonia, anxiogenesis and cognitive dysfunction can be observed reliably. Studies using acute or brief ketamine treatment following withdrawal from CMS have replicated the clinical finding of a rapid onset of antidepressant action. However, there have been no CMS studies of chronic daily ketamine treatment or continued stress following ketamine treatment, which would have greater translational potential in relation to the long-term maintenance of antidepressant effects.
Methods: Wistar rats were drug treated following an initial 2 weeks of CMS exposure, which continued alongside daily drug treatment. A first experiment tested a range of chronic (5 weeks) ketamine doses (5–30 mg/kg); a second compared the effects of subacute (3–5 days) and chronic (5 weeks) treatment.
Results: CMS-induced anhedonic, anxiogenic and dyscognitive effects, as measured, respectively, by decreased sucrose intake, avoidance of open arms in the elevated plus maze and loss of discrimination in the novel object recognition test. A sustained antidepressant-like effect of ketamine in the sucrose intake test was observed in both experiments, with an onset at around 1 week, faster than imipramine, and an optimum dose of 10 mg/kg. Anxiogenic and dyscognitive effects of CMS, in the elevated plus maze and novel object recognition test, respectively, were fully reversed by both subacute and chronic ketamine treatment. Daily treatment with ketamine in the CMS model causes sustained long-term antidepressant, anxiolytic and procognitive effects.
Discussion: The demonstration of a procognitive effect of ketamine may have particular translational value.”
Authors: Mariusz Papp, Piotr Gruca, Magdalena Lason-Tyburkiewicz & Paul Willner
Summary
Introduction
In the 60 years since antidepressant drugs were introduced into clinical practice, there has been little improvement in their efficacy or slow onset of action. A single intravenous infusion of the glutamate N-methyl-D-aspartate (NMDA)-receptor antagonist ketamine has been shown to induce rapid antidepressant effects in treatment-resistant patients.
The effects of ketamine on chronic mild stress-induced anhedonia have been examined in five studies in rats and three studies in mice. All studies used only a single dose of ketamine and the intervening period was always stress-free.
The present study aimed to establish the long-term effects of ketamine on depression and anxiety, to examine the effects of ketamine over a range of behaviourally active doses, and to determine whether ketamine could reverse the cognitive dysfunction caused by stress.
Subjects
Male Wistar rats were transported to the laboratory 1 month before the start of the experiment and were maintained on a 12-h light/dark cycle under conditions of constant temperature (22 2°C) and humidity (50 5%).
Stress procedure
The CMS procedure was implemented as described previously (Papp, 2012), in which animals were first trained to consume a 1% sucrose solution, and then were divided into two matched groups, one of which was subject to CMS.
The stressors included food and water deprivation, 45° cage tilt, intermittent illumination, soiled cage, paired housing, low-intensity stroboscopic illumination, and three periods of no stress.
Elevated plus maze
The apparatus comprised two open arms and two nontransparent closed arms elevated 50 cm above the floor. The animals’ foot movements were recorded manually in a 5-min test.
Novel object recognition test
Rats were tested in an opaque circular open field and were allowed to explore two identical cylinder-shaped white objects for 15 s. A discrimination index was calculated based on the duration of exploration of each object.
Locomotor activity
Locomotor activity was measured in transparent Plexiglas cages equipped with a 15-beam array of infrared horizontal movement detector located 2.5 cm above the floor.
Experiment 1
Experiment 1 tested the effect of ketamine on sucrose intake and behaviour in the elevated plus maze. Animals received once-daily intraperitoneal injections of saline, imipramine or ketamine for a total of 5 weeks.
Experiment 2
Experiment 2 tested the effects of subacute (days) and chronic (weeks) ketamine treatment on sucrose intake in the EPM and novel object recognition test (NORT). Two cohorts of animals were tested: control and stressed animals received once-daily intraperitoneal injections of vehicle (saline, 1 ml/kg) or ketamine (10 mg/kg) for 5 days or 5 weeks.
Statistical analysis
Sucrose intake, EPM, and NORT data were analysed by ANOVA, with the addition of the within-subjects factor treatment duration. Significant interactions were further explored using lower-order ANOVAs and planned comparisons.
Experiment 1
After two weeks of exposure to CMS, imipramine and ketamine increased sucrose intake, but the effect was only apparent at the fifth week of treatment. Ketamine did not significantly restore sucrose intake at higher doses.
In the EPM, stress induced drug interactions were not significant, but CMS decreased open arm entries and open arm time in vehicle-treated animals, and there were significant drug effects on both measures in the CMS group.
Analysis of closed arm entries in the EPM and beam breaks in the actophotometer showed no significant effects of CMS or drug treatment.
Experiment 2
Ketamine treatment induced a time-dependent recovery of sucrose intake in the CMS group, which persisted a week after drug withdrawal. The stress – drug interaction was marginally significant at the 3-day test, but the CMS-ketamine group consumed significantly less sucrose than the control-ketamine group.
In the EPM, CMS decreased open arm entries and ketamine reversed this effect in both the subacute (4-day) and chronic (5-week) cohorts. Ketamine slightly decreased closed arm entries.
CMS abolished discrimination, decreasing the discrimination index to 50%, and ketamine reversed this effect after both subacute and chronic treatment. Ketamine induced a small decrease in the total exploration time, and exploration time was marginally lower in the chronic cohort.
Discussion
Ketamine showed long-term maintenance of effects, an optimal dose, a rapid onset of action, and reversal of anxiogenic and dyscognitive effects of CMS.
Ketamine had an antidepressant-like effect in the CMS model, with a rapid onset, albeit less rapid than has typically been described. The effect was maintained over 5 weeks of daily treatment, in contrast to imipramine, which required 3 weeks of treatment. A randomized clinical trial of repeated ketamine treatment for major depression commenced in August 2016 in Australia and New Zealand.
We found that ketamine had an effect on sucrose intake in rats, but that this effect was lost at higher doses. The loss of effect could be due to a spread of ketamine effects to other brain areas at higher doses.
Ketamine exerted an anxiolytic effect in the EPM, as reported in previous CMS studies using the novelty-suppressed feeding test. The anxiolytic effect may involve a direct projection from the prefrontal cortex to the amygdala, whereas the antianhedonic effect may involve a very indirect multisynaptic projection to the nucleus accumbens.
Ketamine had a procognitive effect in the CMS model and was fully established after subacute (5-day) treatment. Ketamine has been extensively reported to cause cognitive deficits in a variety of tests, including the NORT. However, two recent studies have reported improvements in cognition 72 h following a single infusion of ketamine, suggesting that the procognitive effects of ketamine may rely on different circuitries.
The success of ketamine in clinical studies has focused attention on alternative pharmacological strategies targeting glutamatergic synapses, including NMDA antagonists, partial agonists, and a selective antagonist at the NMDA-1B subunit.
CMS model has been used to study several drugs, including glutamate release inhibitors, glycine transporter inhibitors, and epigenetic inducers of mGlu2 receptors. These drugs have shown clinical effectiveness in treatment-resistant depression.